Bioanalytical Chemistry 22 nd March, 2011

1. DRUG Metabolism and toxicityUmesh M. Hanumegowda MVSc PhD DABTDiscovery ToxicologyBristol-Myers Squibb, Wallingford, CT Bioanalytical Chemistry 22nd March, 2011

2. DRUG METABOLISM Metabolic pathways Metabolizing enzymes & transporters Species differences and Polymorphisms

3. Metabolic Pathways Biotransformation: Conversion of xenobiotics to water-soluble compounds favoring elimination • Phase I • Hydrolysis, reduction, oxidation • Ex., procaine, prontosil, alcohol • Phase II • Glucuronidation, sulfation, methylation, glutathione conjugation, acetylation • Ex., acetaminophen, chloramphenicol, histamine, chlorobenzene, isoniazid

4. Metabolizing Enzymes & Transporters • Microsomal • CYP, FMO, UGT, ALDH, Esterases, Epoxide hydrolases • Mitochondrial • CYP, MAO, ALDH • Cytosolic • NAT, ADH, ALDH, AO, SULT, Esterases, Epoxide hydrolases, GST, Peroxidases • Transporters • MDR, MRP, BCRP, BSEP

5. Species differences & Polymorphisms • Species differences • Ex., Acetylationin dogs, glucuronidation in cats • Ex., Aflatoxintumorigenesis in rats but not in mice • Genetic polymorphisms • Leading to variability • Ex., Slow metabolizers: CYP2D6 ~7% of Caucasians; CYP2C19 ~20% of Asians; FMO3 & fish odor syndrome • Potential for toxicities • Ex., Irinotecan in UGT1A1 deficient population

6. DRUG TOXICITY Classification General scheme Molecular mechanisms with selected examples Adaptation

7. Classification • Exaggerated pharmacology • Ex., Hypotension from beta-blockers • Off-target pharmacology • Ex., QT prolongation with terfenadine • Immunological • Ex., Halothane hepatitis • Reactive metabolites • Ex., Agranulocytosis with clozapine • Idiosyncratic reactions • Ex., Hepatotoxicity with carbamazepine

8. Non-reactive General Scheme of Toxicity DRUG • Exaggerated on-target pharmacology • Off-target pharmacology • Trigger immune response • Concurrent inflammation • Exaggerated on-target pharmacology • Off-target pharmacology Metabolite Reactive Adducted Protein Generation of ROS Altered DNA • Carcinogenesis • Teratogenesis • Loss of function • Enzyme inhibition • Trigger immune response • Deplete cell defense • Trigger cell death pathway • Deplete cell defense • Cell death • Carcinogenesis • Teratogenesis TOX I C I TY From Drug Metabolism Handbook Concepts and Applications

9. Molecular Mechanisms of Toxicity • Loss of function of cellular macromolecules • Covalent modification • Reactivity of intermediate • Examples • Tienilic acid hepatitis • Inactivation of CYP2C9 • Methapyrilenehepatotoxicity • Binding to mitochondrial proteins • NSAIDs liver/intestinal toxicities • Ex., Zomiperac, diclofenacacylglucuronides

10. Molecular Mechanisms of Toxicity • Oxidative stress • ROS (hydrogen peroxide, superoxide, hydroxyl) • Overwhelm cellular defenses • Enzymes (SOD, catalase) reduced glutathione, ascorbate • Oxidative damage of DNA/protein/lipids Monocrotaline Normal liver PMN-induced HOCl modified proteins

11. Molecular Mechanisms of Toxicity 11 • Oxidative stress • Examples • Alcoholic liver disease • ARV and atherosclerosis • DES carcinogenesis • Adenocarcinoma in offspring • Thalidomide teratogenicity • Radical-trapping agent prevent teratogenicity

12. Molecular Mechanisms of Toxicity • Altered balance of cell survival and cell death • p53-dependent apoptosis by disulfiram • Neuronal loss in HIV dementia by NRTI • Acetaminophen toxicity – protection by neutralization of Fasligand/TNF • Immune-mediated • Primarily haptenation • Ex., Autoantibodies to CYP2E1 in halothane hepatitis; Hypersenitivity reactions with abacavir

13. Molecular Mechanisms of Toxicity • Concurrent inflammation • Predispose to toxicity • Idiosyncratic toxicity? • Ex., acetaminophen, ranitidine, chlorpromazine hepatotoxicities precipitated by low-grade inflammation • Kupffer cell depletion protects from acetaminophen toxicity • Inflammatory mediators influence metabolism/toxicity

14. Adaptation • Tachyphyllaxis • Decreased response with subsequent doses • Ex., antidepressants, antipsychotics • Storage • Phospholipidosis with CADs (Ex., Amiodarone) BMS-Y Alveolar Macrophage

15. Adaptation • Enzyme induction • Induction, Autoinduction • Ex., Phenobarbital, Carbamazepine • Relevance to carcinogenesis BMS-X Hypertrophy Normal liver

16. EXAMPLES OF METHODS TO EVALUATE METABOLISM-MEDIATED TOXICITY Metabolic fractions Time-dependent inhibition Metabolism competent cells Reactive metabolite trapping

17. Metabolic Fractions • Liver S9 • Standard for genotoxicity testing ex., Aroclor-induced rat liver S9 in Ames • Microsomes • Hepatic, intestinal, renal • NADPH/ UDPGA fortified • Supersomes • Reaction phenotyping

18. Time-dependent Inhibition • Microsomes • NADPH supplemented • Rate of disappearance of parent/substrate • Ex., Verapamil – moderate time-dependent inhibitor of CYP3A4 (Midazolam as substrate) IC50, T0= 9.3 µM (± 0.7) IC50, T30= 0.7 µM (± 0.07)

19. Metabolism Competent Cells • Primary cells/cell lines • Ex., hepatocytes, renal proximal tubule cells • Limitations ex., Cisplatin in HK2 not predictive • Engineered cells • Individual CYP expressing cells

20. Trapping/ Covalent binding • Glutathione, N-acetyl-cysteine, phenyl-lysine • Epoxides, nitrenium, acyl glucuronide etc., • Potassium cyanide, sodium cyanide • Aldehydes, iminium • Microsomal protein covalent binding

21. Thanks to……….. • Yang Wu • Richard Diters • John Megill • Vinod Arora • Tatyana Zvyaga • Robert Roth • Stephen Adams